How to fix shape keys after having used symmetrize on the basis shape key?
I think it's the same that I just solve.
I had my character with mirror applied and shape keys. I tried to apply the mirror but I cant, so 


The first thing I did is save with another name. 
In the new file I deleted half of my character, leaving the very middle vertex.
In shape key I rename some "Worry" to "WorryL", "Frown" to "FrownL", etc.
In object mode you duplicate my character and made a scale X -1. 
In the new object shape keys rename the renamed ones. "WorryL" to "WorryR", etc
Then I put it in the right position and Alt + J to join. Ctrl + N to fix normals
W and Remove Doubles. 


Ready. 
I have My character With some shapeKeys for Left and Right. Others like Phonemes I leave the like the was.